Mortality of Overwintering Pupae of the Cabbage Root Fly (Delia brassicae)

(1) During the winter of 1976-77, few pupae were removed by predators from either natural or introduced populations of pupae. (2) An increase in pupal density from one to thirty-two pupae/plant did not increase mortality. (3) Over three winters, about 15% of the pupae were killed by physical damage and pathogens. The very cold winter of 1978-79 did not increase mortality. (4) Healthy pupae and pupae parasitized by Idiomorpha rapae were killed by chlorfenvinphos at >5 $\mu$ g g$^{-1}$ of soil at the start of winter. There was a linear relationship between the concentration of chlorfenvinphos and the logit of the proportion of healthy pupae killed. In contrast, the relationship for parasitized pupae, which were extremely sensitive to chlorfenvinphos at concentrations > 10 $\mu$ g g$^{-1}$, was not linear. (5) The percentage of adults emerging from pupae was not affected by the depth of the pupae in the soil. Nevertheless, about 30% of flies that emerged at 3.75 cm to 15 cm and 80% at 30 cm deep failed to reach the soil surface. (6) Cultural practices most affected pupal mortality. When Wellesbourne soil was uncultivated, flies successfully reached the soil surface from nearly half of the total pupae whereas after autumn and spring ploughing only 12 and 31% respectively did so. Ploughing in early winter rather than in spring may therefore aid control of D. brassicae populations.